Osteoporosis is characterized by low bone mineral density (BMD), deterioration of the microarchitecture of bone and subsequent increased fracture. Twin and family studies have suggested that BMD has a strong genetic component, besides being influenced by nutritional and life-style factors. Osteoporosis is regarded as a complex genetic trait, which means that variants of several genes underlie the variability of the phenotype. Among the candidate genes in relation to BMD are the genes for collagen type IA1 (COLIA1), the Vitamin D receptor (VDR) and the estrogen receptor α (ERα). ERα is also known as estrogen receptor I (ESR I). Polymorphisms in the genes for the VDR and the ERα have been examined in relation to BMD. Although contrary reports have been published, two meta-analyses have shown a weak relation between the VDR gene and BMD. We and others have found a significant association between VDR polymorphisms and fracture risk while other studies could not confirm such an association. Also contrary reports regarding the contribution of polymorphisms in the ERα gene to BMD and fracture risk have been published. A recent meta-analysis has shown a relation between the ERα gene and BMD and fracture risk.
The vitamin D receptor gene (12q12) comprises inherited polymorphisms between exon 7 and 3′ UTR of the VDR gene. These alleles are denoted B/b, A/a and T/t for restriction enzyme sites BsmI, ApaI and TaqI respectively (or enzymatic or chemical procedures having similar specificity), where a lower case letter denotes the presence of a wild type restriction site which is capable of being cleaved, and a capital letter denotes the presence of a mutant restriction enzyme site which is not capable of being cleaved by the relevant restriction enzyme. For the purposes of the present invention, determination of which alleles are present in a particular gene may be referred to as determining the genotype of a subject for a particular gene. It is apparent from the above that each copy of the vitamin D receptor gene comprises a specific combination of the three alleles, this combination being referred to as the haplotype of the gene. For example, the haplotype may be baT, indicating the presence of cleavable BsmI and ApaI sites, and a non-cleavable TaqI site. Direct haplotyping of the VDR gene has allowed five different haplotypes to be determined, of which three are common.
Likewise the estrogen receptor α gene (also known as estrogen receptor I) has alleles denoted P/p and X/x for restrictions sites PvuII and XbaI respectively (or enzymatic or chemical procedures having similar specificity), where a lower case letter denotes the presence of a wild type restriction site which is capable of being cleaved, and a capital letter denotes the presence of a mutant restriction enzyme site which is not capable of being cleaved by the relevant restriction enzyme. For the purpose of the present invention, determination of which alleles are present in a particular gene may be referred to as determining the genotype of a subject for a particular gene. It is apparent from the above that each copy of the estrogen receptor α gene comprises a specific combination of the alleles, this combination is referred to as the haplotype of the gene. For example, the haplotype may be px, indication the presence of cleavable PvuI and XbaI sites. Direct haplotyping of the ERα gene has allowed three different haplotypes to be determined.
VDR and ERα are interesting genes because several interactions between the vitamin D and estrogen endocrine system have been described. 1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3) and 17β-estradiol (E2) have a mutual effect on their biosynthesis and receptor expression. Also some genetic studies found an interaction between ERα and VDR genotypes with respect to BMD. Suarez et al. found an interactive effect of ERα and VDR gene polymorphisms on growth in infants.
So far most genetic studies on osteoporosis have focused on BMD as the primary endpoint and not on the clinically more relevant endpoint of fractures. In contrast, the invention disclosed in the present application utilizes the unexpected association between specific ERα and VDR genotypes and the most typical osteoporotic fracture, the vertebral fracture.